The project aims to investigate the nature and properties of the brain hormone(s) of mosquitoes which regulate egg development. Working with Aedes aegypti we have discovered that a hormone from the medial neurosecretory cells of the brain is necessary for egg development to proceed after a blood meal. The target organ of this hormone is the ovary. We have been able to establish a bioassay for the hormone based on its action on the ovary. This bioassay can now be used to purify and characterize the brain hormone and determine its mode of action on the ovary.